Question

A. Describe the spectrum you would observe for the emission spectrum of elemental hydrogen gas.

B. Describe the spectrum you would observe for the absorption spectrum of elemental hydrogen gas.

Answer 1

A) To observe the emission spectrum of elemental hydrogen, when electric discharge is passed through elemental hydrogen, the molecule dissociates into two individual H atoms and emit red light. Only red light is observed because when its four characteristic components - violet, blue, green, red are passed through a prism, the red one at about 656 nm is the most intense and hence, is only observed.

B) To observe the absorption spectrum of elemental hydrogen, shine white light on the gas. The absorption spectrum will be visible as gaps, unlike the lines observed in the emission spectrum. The absorbed photons appear as black lines because the photons of these wavelengths are absorbed and hence, do not appear in the resulting spectrum. As a result, the observed absorption spectrum is inverse of the emission spectrum. This means that the violet, blue, green and red bands will appear as black in the absorption spectrum.